This invention relates to speech compression systems and more particularly to a digital processor which combines the functions of both a vocoder and a modem in a half duplex mode of operation.
Vocoder systems function to transmit speech signals in a coded manner to reduce the transmission bandwidth which would otherwise be required if the speech was to be transmitted in an uncoded manner. Thus a vocoder system includes a transmit terminal to analyze the characteristics of the speech wave to be encoded and to encode the speech wave and a receive terminal which is used to synthesize, from the coded signal sent to it, a reconstruction of the original speech wave. Data modems on the other hand function to facilitate the transmission of data, for example data from a speech vocoder, over a transmission medium. Thus a modern includes a transmit terminal to convert the encoded data into a modulating signal which is used to modulate a carrier frequency, and a receive terminal to demodulate the received signal and thereby recover the transmitted data. Both vocoder and modem equipment are therefore required for transmission of speech signal in an efficient manner.
A known system of particular interest is disclosed in U.S. Pat. No. 3,681,530, entitled "Method and Apparatus for Signal Bandwidth Compression Utilizing The Fourier Transform Of The Logorithm Of The Frequency Spectrum Magnitude", issued to H. J. Manley, et al. There a system is described wherein an input speech waveform is converted into an electrical signal which is then digitized by an analog to digital converter. Following this the digitized signal is directed through a device where magnitudes of the frequency spectrum of the input speech wave are obtained. These magnitudes are then directed to a logging circuit to obtain a logarithm of the frequency spectrum magnitudes of the input speech signal. The log magnitudes of the frequency spectrum are then directed to a computer where the discrete Fourier transform (DFT) of the log spectrum magnitudes are obtained to form what is commonly referred to as the "cepstrum" of the input speech signal. The system also includes an analysis section of the vocoder terminal which operates to decode received data and separate it into pitch data and vocal track impulse data. Also included is a computing device for computing the logarithm of the spectrum envelope of the vocal track impulse response function using FFT techniques. A convolution unit then convolves the pitch data with the impulse response data to yield the desired synthesized speech signal.
A digital modem adapted to be utilized in conjunction with the system described in U.S. Pat. No. 3,681,530 is disclosed in U.S. Pat. No. 3,617,941, entitled "Table Look Up Modulator", which issued to J. DeLellis, Jr., one of the subject inventors, and which discloses a differential phase shift keying (DPSK) system for generating in digital form a plurality of tones at a predetermined baud rate. Both of the foregoing patents are intended to be incorporated herein by reference since they comprise a technical basis for providing a better understanding of the present invention.
Additionally a digital processor which can use the same hardware to implement both the vocoder and modem functions is disclosed in U.S. Pat. No. 3,706,929, entitled "Combined Modem and Vocoder Pipeline Processor", by J. L. Robinson, et al., which describes a processor which operates as a half duplex system, i.e. either in a transmitting or receiving mode. In the "transmit" mode, the input signal to the processor is a speech wave and the processor performs the vecoder function of speech wave analysis and the modem function of generating a modulating signal which carries the results of the speech wave analysis. This modulating signal becomes the output signal of the processor in this mode, which is used in the modulator of a conventional communications transmitting system. In the "receive" mode the input signal to the processor is a speech information bearing communications signal such as may be derived from a conventional communications receiver. The same type of modulation used in the transmit mode must be used in this mode. The processor performs the modem function of demodulating of the input signal and the vocoder function of the synthesis of the speech wave. The output signal of the processor in this mode is then used to drive conventional voice reproduction circuitry.
Acoustic signal processors employing FFT techniques are also disclosed in U.S. Pat. No. 3,662,108, entitled "Apparatus for Reducing Multi-Path Distortion of Signals Utilizing Cepstrum Technique", by J. L. Flannigan and U.S. Pat. No. 3,634,668, entitled "Log Spectrum Decoding Utilizing Symmetry", by H. L. Shaffer.
Accordingly, an object of the present invention is to provide a system including a digital processor which is particularly adapted for voice communications systems.
Another object of the present invention is to provide a system including a digital processor which is adapted to function as both the vocoder and modem in a voice security communications system.
It is a further object of the present invention to provide a system including a digital signal processor which combines the function of both a vocoder and a modem in a half duplex mode of operation.